Abstract: Engineering research has conceptualized and modeled cities as an organismic metabolism, consuming energy and materials, and metabolizing them, and generating emissions and waste. But through this material and energy flow analysis, the specific complex interactions between infrastructure systems that shape these flows remain poorly understood. Understanding how these infrastructures interact with each other and how city-level properties emerge from such underlying interactions is fundamental to the design, development, and operation of sustainable urban systems. This article proposes the concept of infrastructure ecology as a way to analyze, via analogical mapping of urban to natural systems, the complex interdependence of urban infrastructure systems, and offer four fundamental research questions to foster the science of this new concept.